זוהר בראשית — דף קלז ע”א

The Zohar explains that Isaac, severe judgment, and Rebecca, gentle judgment, were like a lily among thorns, and that without Rebecca’s gentleness the world could not bear his judgment; the Holy One, blessed be He, forms couples, one strong and one gentle. Rabbi Yehuda expounds “Isaac entreated the Lord opposite his wife”: Isaac offered a burnt offering and prayed for her, and a fire came down from above to meet the fire below. According to another reading, “entreated” is read as “dug”: Isaac dug upward, to the constellation that governs sons, and the Holy One, blessed be He, received him; then “Rebecca his wife conceived.” Isaac waited twenty years before praying, since the Holy One, blessed be He, accepts the prayers of the righteous.